This invention relates generally to semiconductor lasers and, more particularly, to arrays of semiconductor lasers fabricated as single structures. There are a number of applications of semiconductor lasers that require relatively high output powers, such as space communications, laser printing, and optical recording. In recent years, much of the development effort in semiconductor lasers has been directed to increasing the power output from lasers in continuous wave (cw) operation, both for single laser structures and for phase-locked arrays of multiple lasers.
By way of general background, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes recombine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers in a dielectric waveguide that confines the light in a direction perpendicular to the layers. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in a direction generally perpendicular to the emitting facet.
Various approaches have been used to confine the light in a lateral sense within a semiconductor laser, i.e. perpendicular to the direction of the emitted light and within the plane of the active layer. If a narrow electrical contact is employed to supply current to the device, the lasing action will be limited to a correspondingly narrow region, in a process generally referred to as "gain guiding." At high powers, gain-guided devices have strong instabilities and produce highly astigmatic, double-peaked beams. For most high-power semiconductor laser applications there is also a requirement for a diffraction-limited beam, i.e. one whose spatial spread is limited only by the diffraction of light, to a value roughly proportional to the wavelength of the emitted light divided by the width of the emitting source. Because of the requirement for a diffraction-limited beam, most research in the area has been directed to index-guided lasers. In these, various geometries are employed to introduce dielectric waveguide structures for confining the laser light in a lateral sense, i.e. perpendicular to the direction of light emission and generally in the same plane as the active layer.
A survey of the state of the laser array art can be found in a paper by W. Streifer et al. entitled "Phased Array Laser Diodes," published in Laser Focus/Electro-Optics magazine, June, 1984. A useful introduction to semiconductor lasers and laser arrays can be found in a paper by Dan Botez, entitled "Laser diodes are power-packed," IEEE Spectrum, June, 1985, pp. 43-53.
In general, an array of laser emitters can oscillate in one or more of multiple possible configurations, known as array modes. In the most desirable array mode, all of the emitters oscillate in phase. This is known as the 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe whose width is limited, ideally, only by the diffraction of light. The least desirable array mode, from the standpoint of obtaining a single-lobed far-field pattern, is obtained when adjacent laser emitters are 180.degree. out of phase. This is the 180.degree.-phase-shift array mode, sometimes known as the antisymmetric mode, and it produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters.
There is an inherent tendency for a laser array to oscillate in the antisymmetric array mode. Any array mode having a non-zero field in the lossy regions between laser elements will have a much higher round-trip propagation loss in the laser cavity, compared with a mode which does not have a non-zero field in the regions between laser elements. It is only in the antisymmetric mode that the field diminishes to zero in the regions between lasing elements. Therefore, the propagation loss for oscillation in the antisymmetric mode is lower than for oscillation in the other modes. It is apparently for this reason that the antisymmetric mode emerges as the dominant mode in most laser arrays. Although this mode minimizes propagation losses, it produces a less than desirable far-field pattern, and most research efforts have been directed to structuring the laser array to favor the in-phase mode and to discriminate against the antisymmetric mode. One relatively successful approach is to employ a Y-junction array. However, modification of the laser structure usually introduces more complexity to the fabrication process, and it would be highly desirable to provide an alternative technique for producing a phase-locked array operating in the 0.degree.-phase-shift array mode.
One approach that has been suggested is to coat one of the emitting facets of the array with a film that effects a phase shift in alternate elements of the array. One difficulty with this approach is that the thickness of the film has to be accurately selected and controlled to effect the desired phase shift. Application of the coating is extremely difficult to control, and there is no simple way to adjust the amount of phase shift after the application of the coating.
In view of the foregoing, it will be appreciated that there is still a need for an alternate approach to obtaining a desirable single-lobed far-field radiation pattern from a semiconductor laser array. The present invention fulfills this need.